The present invention generally applies to power regulation applications, and particularly applies to capacitorless DC-to-DC power conversion.
DC-to-DC power conversion typically involves changing a power supply signal from one voltage to another to accommodate the operating voltage requirements of a given load. For example, many computer power supplies provide +24VDC and +12VDC supply output signals, which need to be converted to +5VDC or lower voltage signals for use with the various digital circuits in the computer. Of course, this is just one example of the virtually countless DC-to-DC power conversion applications.
Another common use for DC-to-DC conversion involves converting a battery voltage that changes over time into an essentially constant DC supply signal. This is useful where the circuits in question are optimized for operation at a fixed voltage but maximum battery life is required, e.g., where the discharge curve of the battery must be accommodated.
Two common approaches to DC-to-DC power conversion are linear conversion and non-linear conversion. Linear conversion typically uses some type of transistor as a pass device that may be linearly controlled to introduce a voltage drop from the input supply signal to the regulated output signal. Feedback from the output signal controls the bias on the pass device such that input-to-output voltage drop is varied as needed to maintain the output voltage at the desired voltage level.
Non-linear DC-to-DC power conversion typically uses some type of switching circuit in which one or more reactive components are charged and discharged by switching a supply voltage connection at a varying frequency and/or duty cycle to generate the desired output voltage. While switched-capacitor supplies are used, switched inductor circuits are more commonly applied where appreciable output power is required. Switching regulation usually offers greater efficiency than linear regulation, and oftentimes is the only viable solution in applications that require significant steady-state output power. This fact stems from the excessive power dissipation that arises in a linear regulator""s pass element in applications involving high currents and/or high input-output voltage differentials.
However, switching regulators typically suffer from output noise problems and potentially poor transient response. Output noise is inherent in the on/off switching operation, while transient response is inherently limited by the bandwidth of the switching control loop of the most switching regulators. One approach to reducing these undesirable characteristics involves the use of output capacitance. Low Equivalent Series Resistance (ESR) capacitors placed on the output of a switching regulator serve as a low-impedance reservoir of current, which compensates for switching noise and provides transient current to the load.
However, including low ESR output capacitors in a switching regulator is not without drawbacks. For one, the amount of output capacitance required for satisfactory operation is oftentimes significant, leading to high design costs and significant printed circuit board space usage. Further, depending on the operating voltages involved, finding low ESR capacitors with the necessary voltage rating may be difficult. Other difficulties involve potential surge failures that sometimes plague tantalum capacitors, which are often used in low ESR applications.
The present invention provides methods and apparatus for DC-to-DC power conversion that combines advantages of linear and non-linear power conversion. In an exemplary embodiment, a DC-to-DC converter according to the present invention comprises a parallel combination of linear and non-linear regulators, each providing a regulated output signal that is combined with the other to form a combined regulated output signal. The linear regulator, preferably comprising a linear amplifier circuit, provides most of the transient current required by the load, while the non-linear regulator, preferably a switching regulator, provides most of the steady state and lower frequency current required by the load. In this manner, power dissipation in the linear regulator is minimized, while the need for low ESR output capacitance on the switcher""s output is substantially eliminated.
In at least some exemplary embodiments, the linear amplifier circuit includes an operational amplifier and has push-pull outputs capable of sourcing and sinking load current. The linear amplifier circuit controls its regulated output signal based on feedback taken from the combined regulated output signal, and a reference signal, which may be externally supplied or generated internally. With voltage feedback taken from the converter output, the linear amplifier circuit is made responsive to transient changes at the load, and is thus adapted to provide transient current in response to step changes in required load current. Further, the feedback signal includes ripple noise from the switching regulator, which causes the linear amplifier circuit to generate a compensating AC component on its output signal that acts to minimize overall ripple in the converter""s output signal.
In at least some exemplary embodiments, the switching regulator is driven by a switching control signal that is dependent on sensing the output current of the linear amplifier circuit. A wideband current sensor circuit is preferably used for this purpose, and is driven by the differential voltage signal developed across a current sense element place in the output signal path of the linear amplifier circuit. The current sense signal serves as an input to a comparator circuit, preferably configured as a hysteretic comparator, which uses one or more switching set points that are based on the desired regulation voltage. The comparator generates the switching control signal such that the switching duty cycle and/or switching frequency of the switching regulator is controlled to maintain its output signal at the desired regulation voltage. Because the regulated output signal from the switcher is augmented by the regulated output signal of the linear amplifier circuit, low ESR capacitance is not required on the switching regulator""s output.
Applications in which the inventive converter, in its various embodiments, may be advantageously used are numerous. For example, powering high-performance digital circuitry represents a general application of the converter. Loads of this type are often characterized by rapidly changing power requirements, and demanding input current requirements arising from the high-frequency digital switching that characterizes their operation. Radio base stations, with their abundance of signal processing resources, represent just one of the many types of systems where the converter might be used to significant advantage.